Main vent on basement return. 1 pipe system

STEVEGREENE

#75 Hoffman vent spewing water. Old vent so I replaced. Checked water level and pressure setting as well. Level good 14oz pressure. Worked well for 3 weeks, doing the same thing....I didn’t check firing rate, although I don’t suspect that as issue. Plugged return ??

CantabHeat

Either that or the pressure is actually higher and you’re pushing water back up the return and out the vent. If boiler is in need to a good skim or cleaning water can get pushed back out the return quite easily.

You could take the vent off and pour water down into it. If the return is clogged then water should back up quickly. If not then would more suspect the above.